During excitation-contraction (e-c) coupling Ca2+ release from the sarcoplasmic reticulum (SR) in atrial myocytes differs significantly from ventricular myocytes. Atrial myocytes lack transverse tubules and have two different types of SR, junctional (j-SR) and non-junctional SR (nj-SR). Ca2+ release during e-c coupling is spatially inhomogeneous. Ca2+-induced Ca2+-release (CICR) from j-SR and nj-SR is regulated by distinctly different mechanisms. In atrial myocytes IP3-dependent Ca2+ signalling modulates Ca2+ signaling during e-c coupling and cardiac contractility is regulated by the autonomic nervous system. Beta-adrenergic receptor (betaa-AR) signaling mediates symapathetic regulation of cardiac function through intracellular signaling pathways involving G-proteins, protein kinases, nitric oxide (NO) and Ca2+. Ca2+ alternans reflects the alternations of the Ca2+ transient amplitude at regular pacing frequency which results in electromechanical alternans. Atrial Ca2+ alternans are directly related to the generation of atrial arrhythmias which is a major contributor to cardiovascular morbidity and mortality. The overall goal of the proposed study is to elucidate mechanisms and signalling pathways that are relevant to normal atrial e-c coupling and their perturbations which lead to Ca2+ alternans and therefore arrhythmogenic behavior in atrial tissue. The following Specific Aims are proposed: Specific Aim #1: Determine the subcellular mechanisms by which inositol-phosphate (IP3) signaling governs Ca2+ signaling during e-c coupling. Specific Aim #2. Determine the mechanisms by which a-adrenergic signaling regulates Ca2+ release from j-SR and nj-SR during e-c coupling. Specific Aim #3. Elucidate the mechanisms through which disturbance(s) of IP3-, a-AR- and NO-dependent signaling leads to Ca2+ aiternans. To achieve these aims a multitude of experimental techniques will be used: high resolution [Ca2+]i imaging by laser scanning confocal microscopy in single atrial myocytes, whole-cell voltage clamp techniques to study membrane currents, single channel recordings through cardiac SR Ca2+ release channels reconstituted into planar lipid bilayers, subcellular photolysisof caged Ca2+ and IP3, and pharmacological manipulation of a-adrenergic regulation, IP3 signaling and Ca2+ entry, release and uptake. The proposed research will provide fundamental new information on the regulation of atrial e-c coupling and Ca2+ release under normal and altered conditions relevant to atrial arrhythmias.